The use of printed circuit boards to mount electronic circuitry has grown tremendously in the recent past due to the ease of manufacture and the convenient characteristics which such boards offer. In particular, printed circuit boards may be equipped with conductor patterns which allow the board to be plugged directly into a suitable connector. Plug-in circuit boards may be easily removed for repair and maintenance and then replaced without disturbing the rest of the equipment. Thus, printed circuit boards are particularly suited for applications such as telephone switching systems where it is not practicable to shut down an entire system when only a small portion of the circuitry must be removed for maintenance and repair.
Frequently, circuitry on a printed circuit board which has been unplugged must be placed into an initial state to avoid damaging the circuitry when the board is reinserted into a system that is in operation. It may also be desirable in some cases to initially place the circuitry in a "hold" or "busy" state when it is reinserted into an operational system so that the circuitry may be placed in active service under control of automatic equipment or from a central location. For example, trunk circuitry for use in telephone switching systems often incorporates a relay known as a make-busy or lockout relay. The lockout relay, when operated, is used to make the trunk appear busy to a switching machine when either a malfunction occurs in the trunk circuitry or the trunk is to be removed from service for maintenance purposes. When the trunk is in the busy state it cannot be used by the switching equipment, and faulty operation is thereby avoided. The lockout relay typically has a winding connected to a maintenance desk of the system and is operated by placing a momentary current pulse on a relay control lead that is connected to the maintenance desk. In operating, the lockout relay locks operated over a circuit involving its own normally open contact. The operated lockout relay applies an appropriate battery or ground signal to a supervision lead for making the trunk circuit appear busy to any incoming calls.
Some trunk circuits are mounted on printed circuit boards that are operationally plugged into connectors mounted on the trunk frame of a switching system. When such a trunk circuit is removed from its connector after being placed in the busy state (with the corresponding lockout relay operated), the lockout relay, of course, releases since battery and ground are removed from the circuit when it is unplugged from the connector. After repairs or maintenance operations have been performed on the circuit and the circuit is reinserted into the frame connector, it is desirable that the lockout relay be reoperated in order to make the trunk appear busy. Appropriate tests may then be made by maintenance personnel before the trunk is placed back into service. The relay does not reoperate automatically since the signals which originally operated it are no longer present. One method which has been used to operate the relay is to simply manually push the relay armature, thereby causing the relay to operate and lock operated. However, a problem has arisen in many miniaturized trunk circuits, because the relay cannot be physically accessed. Thus it is desirable in this type of circuit to arrange for initialization of the circuitry when the circuit board is plugged into the connector. It is also desirable that the circuitry on the circuit board be under control of the maintenance desk after the board is plugged into the connector and that the initialization apparatus have no further effect on circuit operation.